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Fluvial sediment transport and deposition following the 1991
eruption of Mount Pinatubo
Shannon K. Hayes a,*, David R. Montgomery a, Christopher G. Newhall b
aDepartment of Geological Sciences, University of Washington, Seattle, WA 98195, USAbU.S. Geological Survey, University of Washington, Seattle, WA 98195, USA
Received 24 January 2001; received in revised form 26 June 2001; accepted 28 September 2001
Abstract
The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic
flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily impacted rivers, revealed negligible critical shear
stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles
moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed
substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were
similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between
volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment
supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to
sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive
disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed
recovers and sediment supply is reduced. D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Bedload; Mount Pinatubo; Sediment transport; Volcano; Watershed disturbance
1. Introduction
Post-eruption sediment transport and deposition are
major problems associated with explosive volcanic
eruptions because these processes can cause wide-
spread damage long after eruptions cease. Rivers
impacted by volcanic eruptions have the highest
recorded specific sediment yields (Fig. 1) due to
increased runoff and erosion from hillslopes mantled
with fine-grained tephra, the destruction of stabilizing
vegetation, and accompanying channel changes
(Swanson et al., 1983; Collins and Dunne, 1986;
Leavesley et al., 1989; Smith and Lowe, 1991; Pierson
et al., 1992, 1996; Major et al., 1996). Although high
erosion rates were previously described at several
volcanoes (Segerstrom, 1950, 1960, 1966; Waldron,
1967; Ollier and Brown, 1971), detailed work follow-
ing the 1980 eruption of Mount St. Helens increased
recognition of the potential impacts of explosive erup-
tions on the hydrology of the surrounding landscape
(Lisle et al., 1983; Swanson et al., 1983; Janda et al.,
1984a,b; Collins and Dunne, 1986; Meyer and Martin-
son, 1989; Dinehart, 1998; Simon, 1999; Major et al.,
0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0169 -555X(01 )00155 -6
* Corresponding author. Present address: Forestry Sciences
Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331, USA.
E-mail address: [email protected] (S.K. Hayes).
www.elsevier.com/locate/geomorph
Geomorphology 45 (2002) 211–224
2000). Since then, studies at Galunggung volcano in
Indonesia (Hamidi, 1989; Hirao and Yoshida, 1989);
Usu, Unzen, and Sakurajima in Japan (Watanabe and
Ikeya, 1981; Kadomura et al., 1983; Shimokawa and
Taniguchi, 1983; Chinen and Kadomura, 1986; Miz-
uyama and Kobashi, 1996); Ruapehu volcano in New
Zealand (Cronin et al., 1999; Hodgson and Manville,
1999); Parıcutin in Mexico (Inbar et al., 1994); and
Mayon volcano and Mount Pinatubo in the Philippines
have addressed the hydrologic response following
explosive volcanic eruptions (Rodolfo, 1989; Rodolfo
andArguden, 1991; Pierson et al., 1992, 1996;Major et
al., 1996; Scott et al., 1996a; Umbal, 1997).
Collins and Dunne (1986) documented that in
volcanically disturbed landscapes, hillslope erosion
rates, which constitute a primary sediment supply to
rivers, decrease exponentially as a rill system develops
and stabilizes. Downstream sediment yields also de-
pend on the rate of river recovery as the hillslope se-
diment supply decreases over time after an eruption.
The complicating effects of changing sediment supply
and the extreme sediment concentrations measured in
rivers impacted by volcanic eruptions prevent the
application of conventional engineering analyses to
predict sediment transport rates and downstream yields
following an explosive volcanic eruption (Pearson and
Eriksen, 1994). Here we report observations of the
magnitude, processes, and effects of fluvial sediment
transport and deposition on the Pasig–Potrero River
alluvial fan 6 years after the cataclysmic eruption of
Mount Pinatubo. Our analysis shows bedload transport
in volcanically disturbed rivers reflects higher transport
capacities than in their nondisturbed counterparts, and
that arid zone channels provide a better analog for
predicting post-eruption bedload transport due to their
high transport efficiency.
1.1. The 1991 eruption of Mount Pinatubo and
physiographic setting
Mount Pinatubo is located about 100 km northwest
of Manila on the west coast of the island of Luzon as
part of the Luzon volcanic arc (Fig. 2). The volcano
consists of steep, deeply dissected upland slopes
bounded by coalescing, low-gradient alluvial fans built
from products of previous eruptions (Scott et al.,
1996a). The mean annual rainfall at Clark Air Base
(altitude 146 m) is 1950 mm, with 60% of this falling in
July, August, and September (Scott et al., 1996a).
The 1991 eruption of Mount Pinatubo deposited
5–6 km3 of pyroclastic material on the flanks of the
volcano, filling adjacent river valleys with up to 200
m of volcanic debris (Scott et al., 1996b). Within 3
years, rapid river re-incision had transported a third of
the volcanogenic sediment downstream (Janda et al.,
1996), depositing it on densely populated alluvial fans
(Table 1). Initially, most sediment from Mount Pina-
tubo was transported in lahars, hyperconcentrated to
debris flows of volcanic material (Janda et al., 1996).
However, as hillslopes stabilized and revegetated and
the channel network reestablished, the threshold rain-
Fig. 1. Annual specific sediment yield versus watershed area of the
Pasig–Potrero River draining Mount Pinatubo in 1991–1995
(PHIVOLCS, personal communication, 1998), the Toutle River
draining Mount St. Helens, 1980–1984 (Dinehart, 1998), and the S.
Cikunir River draining Galunggung volcano in Indonesia, 1982–
1985 (Hirao and Yoshida, 1989), indicating significantly higher
sediment yields in the first 4 years following explosive volcanic
eruptions than in 280 rivers not impacted by volcanic eruptions
(Milliman and Syvitski, 1992). Values plotted for Pinatubo and
Galunggung were calculated from annual accumulated deposit
volumes converted using average bulk deposit densities of 1.3 g/
cm3, and underrepresent the total sediment yields since they are
based solely on terrestrial overbank deposits.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224212
fall necessary for lahar initiation increased (Abigania
et al., 1998), resulting in an increased proportion of
sediment transported by normal river flow.
The Pasig–Potrero River is one of several major
rivers draining the east side of Mt. Pinatubo and is one
of three rivers, including the Sacobia and Abacan
Rivers, draining the Sacobia pyroclastic fan (Fig. 2).
In 1991, pyroclastic flows buried 33% of the Pasig–
Potrero watershed with 0.3 km3 of lithic sand and
pumice, filling the valley to an average depth of 50 m
(Major et al., 1996; Scott et al., 1996b). Since the
cataclysmic eruption in June 1991, secondary explo-
sions in hot pyroclastic-flow deposits and their asso-
ciated secondary pyroclastic flows further altered
watershed geometry (Umbal, 1997). In October
1993, the Pasig–Potrero River captured a large por-
tion of the Sacobia River watershed, enlarging the
Pasig–Potrero upland from 21 to 44 km2, and greatly
increasing the amount of pyroclastic material avail-
able for it to transport.
Upon emerging from the upland at a well-defined
fanhead, the Pasig–Potrero River flows across a low-
gradient alluvial fan composed primarily of older
lahar deposits (Fig. 2). On the upper fan, the channel
is about 100 m wide and has incised up to 20 m into
unconsolidated lahar and fluvial deposits below the
pre-eruption surface in many places. Down-fan, the
channel is wider and less incised.
1.2. Sediment transport measurements
We measured sediment transport rates during the
1997 and 1998 rainy seasons at three sites in the Pasig–
Potrero River at and downslope of the alluvial fanhead
Fig. 2. This 1991 SLAR image (courtesy of Intermap Technologies) of the east side of Mount Pinatubo shows the extent of valley-filling
pyroclastic flows (outlined in black) and of lahar deposition along the Pasig–Potrero River through 1997 (shaded dark gray). Sample sites (1)
Delta 5, (2) the Angeles–Porac Road, and (3) the Transverse Dike are labeled along the 1997 trace of the Pasig–Potrero River (thin dashed
line). A heavy dashed line marks the upper boundary of the coalescing alluvial fans.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224 213
(Fig. 2; Table 2). The primary sampling site was Delta
5 lahar watch point, 16 river km east of the crater at the
head of the Pasig–Potrero alluvial/lahar fan. At Delta
5, the Pasig–Potrero is an incised, braided, sand- to
gravel-bedded river about 150 m wide; it has a mean
slope of 0.020 determined from a least squares linear
regression of a surveyed long profile.
We sampled sediment by the equal-width increment
method, which is recommended for sampling shallow
streams with an unstable cross-sectional discharge
distribution (Edwards and Glysson, 1988), using a
US Geological Survey DH-48 sampler with a 1/4-in.
(0.64 cm) nozzle for suspended sediment and a USGS
Elwha handheld pressure difference sampler (20� 10
cm aperture opening) with a 1-mm mesh bag for
bedload. At discharges >25 m3/s, sediment sampling
was limited to suspended sediment samples taken from
or very near the bank. Bedload discharge was calcu-
lated assuming 100% sampler efficiency (D. Childers,
USGS, personal communication, 1997). Owing to the
spatial and temporal variability of channel morphology
observed at the sample reaches, we often sampled
bedload in individual braids rather than across the
whole channel. We divided each braid into 4 to 15
equally spaced cross-sectional segments and collected
samples from the middle of each segment using a 5-s
sampling interval. Each set of bedload measurements
was repeated four times to minimize error caused by
short-term temporal variation, and the sample closest
to the average wet mass was used for laboratory
analysis. All samples were dried and weighed, and
the bedload samples were sieved to remove particles
< 1 mm in diameter, the size of the sampler mesh.
Suspended-sediment concentration was determined by
filtering the samples and then dividing the weight of
the dried sediment by the volume of the total sample.
Bedload and suspended-sediment fluxes were then
correlated to discharge to produce sediment rating
curves.
We estimated discharge by measuring channel
width, depth, and surface velocity at the beginning
and end of each sampling period. We calculated aver-
age surface velocity from the mean travel times of an
object floated a known distance along three to four
flow paths across the channel, then multiplied the
average surface velocity by 0.8 to obtain the average
velocity for calculating discharge (Matthes, 1956).
Although we realize our velocity estimates are crude,
more accurate means of measuring velocity were not
available; the high sediment load and large number of
rocks rolling in the flow negated use of a cup meter,
pressure bulb, or more precise instruments and the
substantial amount of magnetite in the sediment pre-
vented use of our sturdier propeller current meter with a
magnetic counter. The uncertainty on measurements of
channel width ( ± 1 m), depth ( ± 0.02 m), and velocity
( ± 0.1 m/s) are relatively small; therefore discharge
estimates are likely within 15–20% of the actual
discharge. Discharges >25 m3/s were estimated from
the channel bank because it was impossible to enter the
channel and such measurements are therefore much
more uncertain. During these higher flows, we meas-
ured flow depth near the channel bank and estimated
channel-averaged flow depth visually. We confirmed
visual estimates after flow subsided by measuring the
diameters of large rocks previously observed rolling in
the flow. We calculated surface velocity from floating
objects and measured the total incised channel width
before and after the peak flow.
2. Results
Sediment transport rates in the Pasig–Potrero Ri-
ver remained high during the 1997 and 1998 rainy
seasons. Even at very low flow, the water was turbid
and opaque and submerged portions of the riverbed
were continuous moving carpets of mobile grains. At
Table 1
Rainfall and lahar deposit volumes for the Pasig–Potrero catchment
1991–1997
Year Annual rainfalla (mm) Lahar deposit volume (106 m3)b
1991 2250 50
1992 2200 40
1993 2500 55
1994 2850 140c
1995 2500 90
1996 2000 30
1997 1100 25
a 1991–1993 data from PI2, MSAC, FNG rain gauges (Janda
et al., 1996), 1994–1997 data from upper Sacobia rain gauge
(Abigania et al., 1998).b PHIVOLCS, personal communication, 1998.c The increase in sediment yield in 1994 reflects stream piracy
of the upper Sacobia watershed that almost doubled the Pasig–
Potrero watershed area in October 1993.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224214
all flow stages, we observed larger grains protruding
from the flow, rolling downstream as bedload, and
periodic hydraulic bores like those observed in other
steep shallow channels with fine-grained, non-cohe-
sive, mobile beds (e.g. Fahnestock, 1963; Foley and
Vanoni, 1977; Schumm et al., 1982; Grant, 1997).
At discharges < 25 m3/s, the river was braided with
individual anabranches ranging from shallow, wide
channels to deeper, narrow channels with upstream-
migrating and stationary standing waves separated by
low-amplitude bars (Fig. 3A). During storms, flow
tended to rise as individual bores; as flow approached
25 m3/s, water covered the entire incised channel, and
the bed reorganized into channel-wide dunes (Fig.
3B). At these moderate-flow conditions, bank scour
was substantial, rocks up to 1 m in diameter rolled
downstream, and bores passed every 20–50 s.
The median grain sizes of the bed surface and
subsurface material, determined from pebble counts
and bed material samples collected at Delta 5, were
9.8 and 2 mm, respectively. These measurements
reflect a fine-grained bed with poorly developed
armoring. Fifty-five percent of the bed surface grains
>2 mm in diameter were sub- to well-rounded pumice
particles, with the remaining 45% being angular frag-
ments of lithic material. Both grain size and the
proportion of lithic material on the bed surface
decreased down-fan (Table 2).
The channel bed is not only fine-grained but rela-
tively smooth. Roughness values for the three sampled
reaches of the Pasig–Potrero River calculated using
Manning’s equation are similar to values from low-
slope, sand-bedded rivers (Simons and Simons, 1987)
but are very low relative to other channels with com-
parable slopes (Fig. 4).
2.1. Bedload transport
Measured bedload transport rates in the Pasig–
Potrero River at the head of the alluvial/lahar fan were
as high as 93 kg/s for discharges less than 11 m3/s.
Total bedload flux (Qb) correlated well with dis-
charge, producing a bedload transport rating curve
(Fig. 5) with a least squares regression equation of
Qb ¼ 7:9 Q0:88 ð1Þ
(n = 49, R2 = 0.82, standard error of the estimate =
0.18). Bedload consisted of a mixture of pumice
Fig. 3. Flow stages in the Pasig–Potrero River showing bed reorganization from a braided channel (A) to a fully submerged bed with channel-
wide dunes (B) as flow rises. The channel is about 80 m wide and flow is from right to left.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224 215
particles with an average saturated density of 1.2 ± 0.2
g/cm3 and lithic fragments with an average saturated
density of 2.3 ± 0.2 g/cm3. Although many more
pumice grains were in transport, lithic grains accoun-
ted for a mean 43% of the weight of particles >16 mm
in the bedload samples. The grain size of bedload
samples in the 50th weight percentile ranged from 2 to
17 mm with an average of 5 mm. The aperture size of
the bedload sampler limited collection to particles less
than 10� 20 cm, but particles >10 cm in diameter
covered only 1% of the bed surface.
Scatter in the bedload rating curve results from the
variability inherent to bedload transport (Gomez, 1991)
compounded by active braiding (Davies, 1987), error
in estimating discharge, and error in measuring bed-
load. During the approximately 20-min sampling peri-
ods, braids sometimes widened, filled, or shrank as
flow was diverted. The amount of sediment collected
by the bedload sampler was probably influenced by
local channel changes, such as antidune migration,
upstream bank collapse, and the movement of large
rocks. Normalization of subgroup standard deviations
relative to their respective means for each set of four of
channel-wide bedload measurements used to obtain a
single bedload data point, indicated an average 20%
variability per sample.
Bedload transport is typically considered to be a
function of excess shear stress (Gomez and Church,
1989)
qb ¼ aðs0 � scÞb ð2Þ
where qb is the unit bedload transport rate that occurs
when the effective basal shear stress (s0) exceeds thecritical shear stress required to mobilize the bed (sc).The effective basal shear stress acting on individual
grains (s0) is the difference between the total basal
shear stress (so) (the product of fluid density, gravita-
tional acceleration, average flow depth, and the ener-
gy slope) and the shear stress dissipated by other
forms of roughness, such as bedforms. Critical shear
Fig. 4. Relation between Manning’s roughness coefficient and slope
for the Pasig–Potrero River (open circles) and other mountain
drainage basins reported by Barnes (1967) and Marcus et al. (1992)
(filled circles). The Pasig–Potrero River data were collected at the
study reaches described in Table 1.
Table 2
Pasig–Potrero alluvial fan sediment sampling sites
Sample site Delta 5 Angeles–
Porac Road
Transverse
Dike
Distance downstream
from crater (km)
16 24 36
Reach average slope 0.020 0.009 0.001
Median bed surface
grain size (mm)
9.8 < 2 < 2
Fig. 5. Bedload discharge rating curve for the Pasig–Potrero River
at Delta 5. Data collected during the 1997 and 1998 rainy seasons.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224216
stress (sc) can be determined from a plot of bedload
transport versus total basal shear stress (Fig. 6).
Measurements of unit bedload discharge ( qb) and
total basal shear stress (so) at Delta 5 are well
described by the linear function:
qb ¼ 0:06s0 � 0:001 ð3Þ
(n = 49, R2 = 0.64, standard error of the estimate =
0.33). Setting qb equal to zero in Eq. (3) indicates a
critical shear stress < 0.02 N/m2, corresponding to a
flow depth < 1 mm for this reach of the Pasig–Potrero
River. This small flow depth is not significantly
different from zero, implying that there is effectively
no minimum threshold discharge for particle motion.
2.2. Suspended load transport
We measured suspended sediment concentrations
of 9–293 kg/m3, up to about 20% by volume, at
discharges of 1.2–66 m3/s in the Pasig–Potrero River
at Delta 5. Suspended-sediment concentration and
discharge demonstrated a strong positive correlation
but exhibited the typical scatter of concentration
versus discharge graphs (Fig. 7). In logarithmic space,
the relation between suspended sediment concentra-
tion (C) and discharge (Q) can be described by the
least squares regression equation
C ¼ 9:5Q0:76 ð4Þ
based on 71 composite suspended sediment samples
(R2 = 0.78, standard error of the estimate = 0.18). The
standard deviations of each population of 4 to 10
measurements that composed a single suspended sedi-
ment sample averaged 12%. Though discharge only
describes 78% of the variability in suspended sediment
concentration, less than that in the bedload rating curve,
the low variance within individual samples suggests
that the suspended sediment is less subject to sampling
error and the scatter seen in Fig. 7 represents the natural
variability of sediment supply/transport and error in
discharge estimations, particularly in the upper 20% of
the data points at discharges >25 m3/s.
2.3. Sediment deposition and alluvial fan growth
Two scales of sediment deposition actively shaped
the Pasig–Potrero alluvial fan: aggradation of the
incised channel bed by fluvial deposition and wide-
spread deposition by lahars. These processes differed in
magnitude, duration, and depositional location. During
the 1997 and 1998 rainy seasons, fluvial sediment
transport and deposition was limited to the incised
channel and acted continuously, with most sedimentFig. 6. Bedload flux versus total basal shear stress at Delta 5, 1997–
1998 indicating negligible critical shear stress.
Fig. 7. Suspended sediment concentration versus discharge for the
Pasig–Potrero River at Delta 5, 1997–1998.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224 217
transported during small thunderstorms that occurred
almost daily. Lahar transport occurred much less fre-
quently, yet accounted for the majority of the total
sediment yield of the Pasig–Potrero River (Hayes,
1999).
Measured fluvial sediment transport rates at three
locations on the Pasig–Potrero fan 16, 24, and 36 km
from the crater showed a significant decrease in low-
flow sediment transport down-fan (Fig. 8). Low-flow
transport rates normalized by the amount of sediment
transported onto the alluvial/lahar fan past Delta 5 (km
16) indicated that 20% of the total sediment (suspen-
ded plus bedload) transported onto the fan remained
Fig. 8. Spatial analysis of sediment transport rates shows a down-
fan decrease in transport. Each connected data set (black lines)
represents decreasing transport rates measured consecutively down-
stream at the three locations in a single day. Gray lines representing
suspended sediment concentration indicate that there is no down-fan
change in suspended sediment concentration. Long profile of the
Pasig–Potrero riverbed provided by PHIVOLCS.
Fig. 9. Time-sequential photographs at Delta 5 illustrating � 5 m of
low-flow bed aggradation from August 1997 to September 1998.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224218
mobile past km 24, and only 14% was in motion at the
Transverse Dike. The decrease in suspended sediment
transport downstream was primarily a function of
decreased discharge down-fan due to infiltration of
water into the fan and not caused by a systematic
decrease in suspended-sediment concentration down-
fan. Hence, � 80% of the sediment transported onto
the fan by fluvial processes was deposited within the
channel on the upper fan.
Measurements of bed aggradation support the con-
clusion that most fluvially transported sediment was
deposited on the upper fan. Bed aggradation docu-
mented with a gauge painted on the channel wall at
Delta 5 indicated 1.9 m of low-flow aggradation in 4
weeks in 1997 and 2.2 m in 5.5 weeks in 1998 (Fig. 9).
The net change in channel bed elevation at Delta 5 from
August 22, 1997, through September 21, 1998, was
� 5 m. No noticeable change in bed elevation occurred
at the Transverse Dike either year. The majority of this
aggradation occurred during moderate flows (� 30–
100 m3/s) caused by small thunderstorms and took
place entirely within the incised channel.
Lahar deposition was considerably more variable.
Although lahars occur only a few times a year during
large storms, they transport and deposit enormous
volumes of sediment; overbank lahar deposits blanket
a large portion of the mid to lower fan. Lahar de-
positional zones have migrated up and down the fan
resulting in substantial temporal variation in bed
elevation at the alluvial fanhead on the order of tens
of meters. During the 1997 and 1998 rainy seasons,
fluvial sediment transport resulted in bed aggradation
at the alluvial fanhead, whereas lahars incised the
fanhead and deposited sediment downstream on the
middle to lower fan. Stratigraphic evidence from a
lahar in August 1997 indicated initial aggradation of
the channel bed at Delta 5 of � 4 m, followed by at
least 18 m of erosion, resulting in a net bed elevation
loss of 14 m from the pre-storm channel bed.
3. Discussion
3.1. Similarity between arid and volcanically dis-
turbed rivers
Comparison of dimensionless bedload transport
rates to dimensionless shear stress incorporated the
effects of different sediment and fluid densities and
bed grain size on bedload transport and allowed us to
compare transport rates measured in the Pasig–
Potrero River to those of other rivers (Fig. 10). Di-
mensionless bedload transport rates in the Pasig–
Potrero River were significantly higher than in rivers
with more uniform fine-grained beds, e.g. the sandy
East Fork River in Wyoming (Emmett et al., 1980,
1985) and coarser, gravel-bedded rivers like Oak
Creek in Oregon (Milhous, 1973) for the same shear
stress. Yet rates measured in the Pasig–Potrero River
were comparable to those measured during flash
floods in the ephemeral Nahal Yatir in Israel (Reid
et al., 1995) and in the North Fork Toutle River
following the 1980 eruption of Mount St. Helens
(Pitlick, 1992). The differences in transport rates
cannot be explained by simple trends in grain size or
slope (Table 3).
The high bedload transport rates measured in the
Pasig–Potrero River were due to the combined
effects of an enormous supply of easily mobilized,
fine-grained sediment and changes in the bed com-
position and morphology that affected the river’s
transport efficiency. The massive input of sediment
by the 1991 eruption of Mount Pinatubo reduced the
bed grain size and overall roughness of the Pasig–
Potrero River by burying the pre-existing channel in
the upper watershed under thick deposits of fine-
grained pyroclastic material (Major et al., 1996).
Post-eruption deposition of this sediment produced
a wide, flat braidplain devoid of vegetation with a
relatively fine-grained, smooth bed with roughness
values typical of sand-bedded rivers yet on much
steeper slopes than sand-bed channels are typically
found.
The eruption supplied far more sediment than the
river has been able to transport, thus ‘‘swamping’’ the
channel and preventing development of a coarse sur-
face layer (Dietrich et al., 1989; Lisle and Madej,
1992). Recent studies document higher bedload effi-
ciency over smooth beds in arid, ephemeral streams
than in channels with coarser-grained surface layers
(Laronne and Reid, 1993; Reid and Laronne, 1995)
and over sand portions of mixed-bed channels (Fer-
guson et al., 1989). Furthermore, where gravel and
cobbles are transported over a fine-grained bed, the
protrusion of large particles into the flow increases the
river’s competence, or ability to transport larger par-
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224 219
ticles (Ferguson et al., 1989; Montgomery et al.,
1999).
The fine-grained, smooth, steep bed produced by
post-eruption deposition of sediment probably also
enhanced transport rates by inducing pulsating flow.
Pulsating flow, like the periodic bores witnessed in
the Pasig–Potrero, is most likely caused by the
formation and destruction of antidunes at near-critical
flow and is highly erosive due to a temporary increase
in flow depth and turbulence (Schumm et al., 1982;
Grant, 1997). We observed higher transport rates in
the wake of breaking antidunes and during the pas-
sage of bores. Large, previously stationary rocks were
commonly mobilized and transported short distances
by bores.
As bedload transport reflects a balance between
the forces acting on the bed and the forces resisting
motion (Eq. (2)), a massive disturbance like the 1991
eruption of Mount Pinatubo can alter the bedload
transport regime by changing the basal shear stress
(via changes in flow conditions) or the critical shear
stress (by changing the bed grain size through an
increase in sediment supplied to the channel). While
hydrologic changes that can affect basal shear stress,
such as increased runoff caused by a reduction in
infiltration capacity on hillslopes blanketed with fine-
grained ash, are relatively short-lived, changes to the
channel itself can persist for decades flowing a dis-
turbance (Major et al., 2000). In undisturbed systems,
channel beds typically reflect flow conditions. How-
ever, immediately following a disturbance, channels
do not have time to re-adjust to the increased sedi-
ment load, resulting in finer-grained beds relative to
the pre-disturbance conditions. This in turn causes a
temporary increase in excess shear stress and accel-
erated bedload transport, as seen in channels on
Mount Pinatubo and Mount St. Helens. Flash floods
in arid environments also tend to have high excess
shear stress and therefore high bedload transport rates
because the discharges during flash floods are much
greater than typical flow conditions (Pitlick, 1992).
The high concentrations of suspended sediment meas-
ured in the water column at Mount Pinatubo and in
flash floods may further enhance bedload transport by
increasing the fluid viscosity and density, thus redu-
cing particle settling velocity and increasing basal
shear stress (Bradley and McCutcheon, 1987; Simons
and Simons, 1987).
Fig. 10. Comparison of dimensionless bedload transport rates ( qb*)
against dimensionless shear stress (s*) for the Nahar Yatir (Reid et
al., 1995), the Pasig–Potrero River, the North Fork Toutle River
(Pitlick, 1992), Oak Creek (Milhous, 1973), and two sections on the
East Fork River located at the datum and 3256 m upstream (Emmett
et al., 1980, 1985). Dimensionless bedload transport rate ( qb*) is
given by qb * = qb/qs[ g(qs� q/q)D503]0.5 and dimensionless shear
stress (s*) is given by s * = s0 /(qs� q)gD50, where q and qs are thefluid and sediment densities, g is gravitational acceleration, and D50
is the median grain size of the bed surface (Table 3).
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224220
3.2. Implications of high post-eruption sediment
transport rates
The observation of very efficient bedload transport
has considerable implications for prediction of trans-
port rates following watershed disturbance. Bedload
transport equations developed and calibrated in chan-
nels with armored beds, such as Oak Creek, or even
the same channel prior to an eruption may grossly
underestimate post-eruption bedload transport in
channels having finer-grained, unarmored beds. Chan-
nels in highly disturbed systems undergo morphologic
and hydrologic changes that make them more efficient
transporting agents, which recover to more stable
systems as the sediment supply is reduced. The
extreme sediment transport rates observed in the
Pasig–Potrero River reflect a positive feedback loop
in which the river’s transport capacity adjusted to the
sediment supply through reduced bed-surface grain
size and roughness, which in turn generated faster,
shallower, sediment-laden, near-critical flow with
high transport efficiency. Higher transport efficiency
allowed for removal of more sediment, which helped
reduce the overall sediment supply, thus speeding
watershed recovery.
Sediment transport following major explosive
eruptions can have persistent, far-reaching effects as
sediment is carried downstream into populated areas.
In 1997, normal streamflow transported � 11 million
mg of sediment to the Pasig–Potrero alluvial fan,
accounting for roughly 25% of the total annual yield
but still two orders of magnitude more than the pre-
eruption sedimentation levels in spite of 1997 being a
dry year (JICA, 1978; Hayes, 1999). The high fluvial
transport rates documented here lead to substantial
deposition on the Pasig–Potrero alluvial/lahar fan-
head. Channel bed aggradation reduces conveyance;
this loss of channel capacity can enhance the effects of
small floods, induce lateral bank erosion and thereby
threaten dikes, and increase the potential for channel
avulsion during high-flow events. Because recent
lahars incised the fanhead while fluvial transport
aggraded the channel bed, the sequence of erosion
and deposition, in addition to the magnitude of bed
aggradation, may be an important factor in determin-
ing where and when channel avulsion will occur.
Rivers draining volcanoes are capable of trans-
porting very high sediment loads, similar to loads
transported by river in arid climates during flash
floods. Yet unlike an arid environment, volcanically
disturbed rivers are commonly subject to tropical
rainfall. The vast sediment supply and rapid hydro-
logic response characteristic of an arid environment
combined with tropical rainfall is a recipe for extreme
sediment yields, such as those experienced at Mount
Pinatubo.
Acknowledgements
We thank Dr. R.S. Punongbayan and the Philippine
Institute of Volcanology and Seismology for generous
scientific and logistical support. We are grateful to
Maria Panfil, Harriet Shields, and Mari Bingham for
field assistance and the late Dallas Childers (USGS)
for his advice on sediment sampling techniques.
Insightful critiques by Gordon Grant, James Knox, W.
Andrew Marcus, Jon Major, Jim O’Connor, and an
anonymous reviewer greatly improved the manu-
script. This research was funded in part by the US
Geological Survey/USAID Volcano Disaster Assis-
tance Program.
Table 3
Hydraulic and bed characteristics of channels used in bedload transport rate comparison
Nahal Yatir,
Israel
Pasig–Potrero,
Mt. Pinatubo
North Fork
Toutle River,
Mt. St. Helens
Oak Creek,
Oregon
East Fork
River 0000
East Fork
River 3256
Sampling method Birkbeck-
type slot
Helley–Smith-
type (Elwha)
Helley–Smith-
type (TR-2)
Birkbeck-
type slot
Conveyer-
belt trap
Helley–
Smith
Average water surface slope 0.009 0.020 0.008 0.014 0.001 0.001
Bed surface D50 (mm) 6 9.8 � 30 60 1.4 6.8
Subsurface D50 (mm) 10 2 N.D. 20 N.D. N.D.
Bedload D50 (mm) 6 5 14 48 1 0.68
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224 221
References
Abigania, M.I.T., Arboleda, R.A., Delos Reyes, P.J., Panol, M.D.,
Tungol, N.M., 1998. The Pasig –Potrero River/lahar system:
assessment and prognosis for the 1998 rainy season. Philippine
Institute of Volcanology and Seismology, internal report.
Barnes, H.H., 1967. Roughness characteristics of natural channels.
U.S. Geological Survey Water-Supply Paper 1849, 213 pp.
Bradley, J.B., McCutcheon, S.C., 1987. Influence of large sus-
pended-sediment concentrations in rivers. In: Thorne, C.R.,
Bathurst, J.C., Hey, R.D. (Eds.), Sediment Transport in Grav-
el-bed Rivers. Wiley, NY, pp. 645–689.
Chinen, T., Kadomura, H., 1986. Post-eruption sediment budget of a
small catchment on Mt. Usu, Hokkaido. Zeitschrift fuer Geo-
morphologie, Supplementband 60, 217–232.
Collins, B.D., Dunne, T., 1986. Erosion of tephra from the 1980
eruption of Mount St. Helens. Geological Society of America
Bulletin 97 (7), 905–986.
Cronin, S., Neall, V., Lecointre, J., Palmer, A., 1999. Dynamic
interactions between lahars and stream flow; a case study from
Ruepehu Volcano, New Zealand. Geological Society of America
Bulletin 111 (1), 28–38.
Davies, T.R.H., 1987. Problems of bedload transport in braided
gravel-bed rivers. In: Thorne, C.R., Bathurst, J.C., Hey, R.D.
(Eds.), Sediment Transport in Gravel-bed Rivers. Wiley, NY, pp.
793–841.
Dietrich, W.E., Kirchner, J.W., Ikeda, H., Iseya, F., 1989. Sediment
supply and the development of the coarse surface layer in grav-
el-bedded rivers. Nature 340 (6230), 215–217.
Dinehart, R.L., 1998. Sediment transport at gauging stations near
Mount St. Helens, Washington, 1980–90. Data collection and
analysis. U.S. Geological Survey Professional Paper 1573, 105
pp.
Edwards, T.K., Glysson, G.D., 1988. Field methods for measure-
ment of fluvial sediment. U.S. Geological Survey Open-File Re-
port 86-531, 118 pp.
Emmett, W.W., Myrick, R.M., Meade, R.H., 1980. Field data de-
scribing the movement and storage of sediment in the East Fork
River, Wyoming: Part I. River hydraulics and sediment transport,
1979. U.S. Geological Survey Open-File Report 80-1189, 43 pp.
Emmett, W.W., Myrick, R.M., Martinson, H.A., 1985. Hydraulic
and sediment-transport data, East Fork River, Wyoming, 1978.
U.S. Geological Survey Open-File Report 85-486, 37 pp.
Fahnestock, R.K., 1963. Morphology and hydrology of a glacial
stream–white river, Mount Rainier, Washington. U.S. Geolog-
ical Survey Professional Paper 422 A, 69 pp.
Ferguson, R.I., Prestegaard, K.L., Ashworth, P.J., 1989. Influence of
sand on hydraulics and gravel transport in a braided gravel bed
river. Water Resources Research 25 (4), 635–643.
Foley, M.G., Vanoni, V.A., 1977. Pulsing flow in steep alluvial
streams. Journal of the Hydraulics Division, American Society
of Civil Engineers 103 (2), 843–850.
Gomez, B., 1991. Bedload transport. Earth-Science Reviews 31 (2),
89–132.
Gomez, B., Church, M., 1989. An assessment of bedload sediment
transport formulae for gravel bed rivers. Water Resources Re-
search 25 (6), 1161–1186.
Grant, G.E., 1997. Critical flow constrains flow hydraulics in mo-
bile-bed streams: a new hypothesis. Water Resources Research
33 (2), 349–358.
Hamidi, S., 1989. Lahar of Galunggung Volcano from 1982 through
1986. Proceedings of International Symposium on Erosion and
Volcanic Debris Flow Technology, July 31–August 3. Ministry
of Public Works, Yogyakarta, Indonesia, pp. V1-1–VP1-23.
Hayes, S.K., 1999. Low-flow sediment transport on the Pasig–Po-
trero alluvial fan, Mount Pinatubo, Philippines. MS Thesis, Uni-
versity of Washington, Seattle.
Hirao, K., Yoshida, M., 1989. Sediment yield of Mt. Galunggung
after eruption in 1982. Proceedings of International Symposium
on Erosion and Volcanic Debris Flow Technology, July 31–
August 3. Ministry of Public Works, Yogyakarta, Indonesia, pp.
V21-1–V21-22.
Hodgson, K., Manville, V., 1999. Sedimentology and flow behavior
of a rain-triggered lahar, Mangatoetoenui Stream, Ruapehu Vol-
cano, New Zealand. Geological Society of America Bulletin 111
(5), 743–754.
Inbar, M., Lugo Hubp, J., Villers Ruiz, L., 1994. The geomorpho-
logical evolution of the Paricutin cone and lava flows, Mexico,
1943–1990. Geomorphology 9 (1), 57–76.
Janda, R.J., Meyer, D.F., Childers, D., 1984a. Sedimentation and
geomorphic changes during and following the 1980–1983 erup-
tions of Mount St. Helens, Washington, I. Shin-Sabo Journal of
the Erosion Control Engineering Society of Japan 37 (2), 10–
21.
Janda, R.J., Meyer, D.F., Childers, D., 1984b. Sedimentation and
geomorphic changes during and following the 1980–1983 erup-
tions of Mount St. Helens, Washington, II. Shin-Sabo Journal of
the Erosion Control Engineering Society of Japan 37 (3), 5–19.
Janda, R.J., Daag, A.S., Delos Reyes, P.J., Newhall, C.G., Pierson,
T.C., Punongbayan, R.S., Rodolfo, K.S., Solidum, R.U., Umbal,
J.V., 1996. Assessment and response to lahar hazard around
Mount Pinatubo, 1991 to 1993. In: Newhall, C.G., Punong-
bayan, R.S. (Eds.), Fire and Mud, Eruptions and Lahars of
Mount Pinatubo, Philippines. PHIVOLCS Press, Quezon City,
and University of Washington Press, Seattle, pp. 107–139.
Japan International Cooperating Agengy (JICA), 1978. Planning
Report on the Pasig –Potrero River Flood Control and Sabo
Project: JICA, Tokyo, Japan.
Kadomura, H., Imagawa, T., Hiroshi, Y., 1983. Eruption-induced
rapid erosion and mass movements on Usu Volcano, Hokkaido.
Zeitschrift fuer Geomorphologie, Supplementband 46, 124–142.
Laronne, J., Reid, I., 1993. Very high rates of bedload sediment
transport by ephemeral desert rivers. Nature 366 (6451), 148–
150.
Leavesley, G.H., Lusby, G.C., Lichty, R.W., 1989. Infiltration and
erosion characteristics of selected tephra deposits from the 1980
eruption of Mount St. Helens, Washington, USA. Hydrological
Sciences 34 (3), 339–353.
Lisle, T.E., Madej, M.A., 1992. Spatial variation in armouring in a
channel with high sediment supply. In: Billi, P., Hey, R.D.,
Thorne, C.R., Tacconi, P. (Eds.), Dynamics of Gravel-bed Riv-
ers. Wiley, NY, pp. 277–293.
Lisle, T.E., Lehre, A.K., Martinson, H.A., Meyer, D.F., Nolan,
K.M., Smith, R.D., 1983. Stream channel adjustments after
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224222
the 1980 Mount St. Helens eruptions. Symposium on Erosion
Control in Volcanic Areas, Proceedings. Technical Memoran-
dum of Public Works Research Institute, vol. 1908. Ministry
of Construction, Japan, pp. 33–72.
Major, J.J., Janda, R.J., Daag, A.S., 1996. Watershed disturbance
and lahars on the east side of Mount Pinatubo during the mid-
June 1991 eruptions. In: Newhall, C.G., Punongbayan, R.S.
(Eds.), Fire and Mud, Eruptions and Lahars of Mount Pinatubo,
Philippines. PHIVOLCS Press, Quezon City, and University of
Washington Press, Seattle, pp. 895–920.
Major, J.J., Pierson, T.C., Dinehart, R.L., Costa, J.E., 2000. Sediment
yield following severe volcanic disturbance—a two-decade per-
spective from Mount St. Helens. Geology 28 (9), 819–822.
Marcus, W.A., Roberts, K., Harvey, L., Tackman, G., 1992. An
evaluation of methods for estimating Manning’s n in small
mountain streams. Mountain Research and Development 12,
227–239.
Matthes, G., 1956. River surveys in unmapped territory. American
Society of Civil Engineers, Transactions 121, 739–758.
Meyer, D.F., Martinson, H.A., 1989. Rates and processes of channel
development and recovery following the 1980 eruption of
Mount St. Helens, Washington. Hydrological Sciences 34 (2),
115–127.
Milhous, R.T., 1973. Sediment transport in a gravel-bottomed
stream. PhD Dissertation, Oregon State University, Corvallis.
Milliman, J.D., Syvitski, J.P.M., 1992. Geomorphic/tectonic control
of sediment discharge to the ocean: the importance of small
mountainous rivers. Journal of Geology 100 (5), 525–544.
Mizuyama, T., Kobashi, S., 1996. Sediment yield and topographic
change after major volcanic activity. Erosion and Sediment
Yield: Global and Regional Perspectives, vol. 236. IAHS Pub-
lication, Wallingford, Oxfordshire, pp. 259–301.
Montgomery, D.R., Panfil, M.S., Hayes, S.K., 1999. Channel-bed
mobility response to extreme sediment loading at Mount Pina-
tubo. Geology 27 (3), 271–274.
Ollier, C.D., Brown, M.J.F., 1971. Erosion of a young volcano in
New Guinea. Zeitschrift fuer Geomorphologie 15 (1), 12–28.
Pearson, M.L., Eriksen, K.W., 1994. Geomorphic and sedimentation
investigation of the 15 June 1991 eruption of Mount Pinatubo,
the Philippines. Technical report U.S. Army Engineer Water-
ways Experiment Station GL-94-14, 106 pp.
Pierson, T.C., Janda, R.J., Umbal, J.V., Daag, A.S., 1992. Immedi-
ate and long-term hazards from lahars and excess sedimentation
in rivers draining Mt. Pinatubo, Philippines. U.S. Geological
Survey Water-Resources Investigations Report 92-4039, 35 pp.
Pierson, T.C., Daag, A.S., Delos Reyes, P.J., Regalado, M.T.M.,
Solidum, R.U., Tubianosa, B.S., 1996. Flow and deposition of
posteruption hot lahars on the east side of Mount Pinatubo,
July –October 1991. In: Newhall, C.G., Punongbayan, R.S.
(Eds.), Fire and Mud, Eruptions and Lahars of Mount Pinatubo,
Philippines. PHIVOLCS Press, Quezon City, and University of
Washington Press, Seattle, pp. 921–950.
Pitlick, J., 1992. Flow resistance under conditions of intense gravel
transport. Water Resources Research 28 (3), 891–903.
Reid, I., Laronne, J., 1995. Bedload sediment transport in an ephem-
eral stream and a comparison with seasonal and perennial coun-
terparts. Water Resources Research 31 (3), 773–781.
Reid, I., Laronne, J., Powell, D., 1995. The Nahal Yatir bedload
database: sediment dynamics in a gravel-bed ephemeral stream.
Earth Surface Processes and Landforms 20 (9), 845–857.
Rodolfo, K.S., 1989. Origin and early evolution of lahar channel at
Mabinit, Mayon Volcano, Philippines. Geological Society of
America Bulletin 101 (3), 414–426.
Rodolfo, K.S., Arguden, A.T., 1991. Rain– lahar generation and
sediment-delivery systems at Mayon Volcano, Philippines. In:
Fisher, R.V., Smith, G.A. (Eds.), Sedimentation in Volcanic Set-
tings. SEPM Special Publication, vol. 45, pp. 71–87. Tulsa,
OK.
Schumm, S.A., Bean, D.W., Harvey, M.D., 1982. Bed-form-de-
pendant pulsating flow in Medano Creek, Southern Colorado.
Earth Surface Processes and Landforms 7 (1), 17–28.
Scott, K.M., Janda, R.J., de la Cruz, E.G., Gabinete, E., Eto, I.,
Isada, M., Sexton, M., Hadley, K.C., 1996. Channel and sed-
imentation responses to large volumes of 1991 volcanic deposits
on the east flank of Mount Pinatubo. In: Newhall, C.G., Pu-
nongbayan, R.S. (Eds.), Fire and Mud, Eruptions and Lahars of
Mount Pinatubo, Philippines. PHIVOLCS Press, Quezon City,
and University of Washington Press, Seattle, pp. 971–988.
Scott, W.E., Hoblitt, R.P., Torres, R.C., Self, S., Martinez, M.M.L.,
Nillos, T., 1996. Pyroclastic flows of the June 15, 1991, climac-
tic eruption of Mount Pinatubo. In: Newhall, C.G., Punong-
bayan, R.S. (Eds.), Fire and Mud, Eruptions and Lahars of
Mount Pinatubo, Philippines. PHIVOLCS Press, Quezon City,
and University of Washington Press, Seattle, pp. 545–570.
Segerstrom, K., 1950. Erosion studies at Parıcutin, State of Michoa-
can, Mexico. U.S. Geological Survey Bulletin 965-A, 1–64.
Segerstrom, K., 1960. Erosion and related phenomena at Parıcutin
in 1957. U.S. Geological Survey Bulletin 1104-A, 1–18.
Segerstrom, K., 1966. Parıcutin, 1965—aftermath of eruption. U.S.
Geological Survey Professional Paper 550-C, C93–C101.
Shimokawa, E., Taniguchi, Y., 1983. Sediment yield from hillslopes
on active volcanoes. Symposium on Erosion Control in Volcanic
Areas, Seattle and Vancouver, WA, July 6–9, 1982, Proceed-
ings, 155–182 Technical Memorandum of Public Works
Research Institute, Ministry of Construction, Japan.
Simon, A., 1999. Channel and drainage-basin response of the Toutle
River system in the aftermath of the 1980 eruption of Mount St.
Helens, Washington. U.S. Geological Survey Open-File Report
96-633, 130 pp.
Simons, D.B., Simons, R.K., 1987. Differences between gravel- and
sand-bed rivers. In: Thorne, C.R., Bathurst, J.C., Hey, R.D.
(Eds.), Sediment Transport in Gravel-bed Rivers. Wiley, NY,
pp. 3–15.
Smith, G.A., Lowe, D.R., 1991. Lahars: volcano–hydrologic events
and deposition in the debris flow-hyperconcentrated flow con-
tinuum. In: Fisher, R.V., Smith, G.A. (Eds.), Sedimentation in
Volcanic Settings. SEPM Special Publication, vol. 45, pp. 60–
70. Tulsa, OK.
Swanson, F.J., Collins, B., Dunne, T., Wicherski, B.P., 1983. Ero-
sion of tephra from hillslopes near Mt. St. Helens and other
volcanoes. Proceedings of the Symposium on Erosion Control
in Volcanic AreasTechnical Memorandum of the Public Works
Research Institute, vol. 1908. Ministry of Construction, Japan,
pp. 183–221.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224 223
Umbal, J.V., 1997. Five years of lahars at Pinatubo volcano: declin-
ing but still potentially lethal hazards. Journal of the Geological
Society of the Philippines 52, 1–19.
Waldron, H., 1967. Debris flow and erosion control problems
caused by the ash eruptions of Irazu Volcano, Costa Rica.
U.S. Geological Survey Bulletin 1241-I, 37 pp.
Watanabe, M., Ikeya, H., 1981. Investigations and analysis of vol-
canic mud flows on Mt. Sakurajima, Japan. Erosion and Sedi-
ment Transport Measurement, vol. 113. IAHS Publication,
Wallingford, Oxfordshire, U.K., pp. 245–256.
S.K. Hayes et al. / Geomorphology 45 (2002) 211–224224